1. Field of the Invention
The invention disclosed herein relates to estimating elemental concentrations and formation lithology of an earth formation and, in particular, to using a logging tool to provide data used for the estimating.
2. Description of the Related Art
Reservoirs of hydrocarbons are generally contained in rock formations. Many types of lithology may include these reservoirs. Factors such as the size and location of the reservoirs may depend on the type of lithology. For example, a particular type of lithology can indicate characteristics consistent with a hydrocarbon-bearing reservoir. Thus, an accurate knowledge of the type of lithology encountered during exploration or production can provide for an efficient use of exploration and production resources.
Well logging is a technique used to measure formation properties. In well logging, a logging tool is conveyed in a borehole penetrating an earth formation. The logging tool contains an instrument adapted for measuring physical values being proportional to the concentrations of the elements that make up the minerals in a lithology of the earth formation. The instrument, such as a spectrometer, can measure gamma-ray spectra to extract from them yields of various elements with varying degrees of accuracy. Thus, some elemental yields will be measured with more accuracy than other elemental yields. Unfortunately, inaccurate determinations of elemental yields can lead to inaccurate determinations of elemental concentrations and of a type of lithology. Without an accurate determination of the type of lithology, petroanalysts may perform inaccurate assessments of reservoir productivity and hydrocarbon reserves.
Therefore, what are needed are techniques to accurately estimate a particular lithology. Preferably, the techniques can be used with data, such as measurements of spectra, obtained from logging tools.